In order to enforce nondestructive testing of a concrete structure by using an elastic wave, it is necessary to precisely detect a reflection wave or a transmission wave propagating through the interior of the concrete. Namely, in view of a magnitude of a bridge, a road, dam, a building or the like to be inspected, a wave transmitter is required to have an ability to effectively injecting, into the measurement object, the elastic wave in the level of a frequency of vibration detectable even if the wave transmits the interior of the concrete for several tens cm to several tens m. Unlike a uniform material such as metal, in case of the concrete, if an acoustic wave in the band of about several MHz to be used for a metal material is introduced into the concrete, the attenuation is severe and the reach distance is short. Thus, it is impossible to use this acoustic wave for the concrete. For this reason, the band from several tens Hz to several tens kHz has to be utilized.
Conventionally, in the nondestructive testing of such a engineering building structure made of concrete, the measurement of the acoustic velocity or a thickness of the measurement object and an inference of an interior structure or a position of an abnormal portion are performed by a impact elastic wave method using an impact hammer, and by supersonic tests such as a pulse reflection method in which a probe using a piezoelectric vibrator is used as a wave transmitter for a supersonic wave, a transmission method and a resonance method.
The piezoelectric material constituting the piezoelectric vibrator that has been extensively used as a wave transmitter for a supersonic wave generates a strain in accordance with a magnitude of an electric field. The piezoelectric vibrator has a structure in which the piezoelectric material is sandwiched by electrodes and outputs a large vibration at a frequency at which its thickness corresponds to a half-wavelength of a longitudinal wave of the piezoelectric material. In general, a mechanical Q value of the piezoelectric material is high and the output efficiency of other than this mechanical resonance point is markedly degraded. Accordingly, in order to obtain a vibration in the band that is required for diagnosis of the concrete structure, the thickness of the piezoelectric vibrator should be several tens cm or more. However, it is very difficult to structure such a large size piezoelectric vibrator.
Namely, the probe using the piezoelectric vibrator is suitable for outputting a supersonic wave having a uniform frequency that is equal to or more than several tens MHz due to its characteristics. For this reason, there is a limit to the measurement in the nondestructive testing of the concrete structure in which the high frequency wave is remarkably attenuated. In particular, since it is difficult to obtain the energy of a low frequency wave that is needed for detecting the supersonic wave propagating through several tens m or longer by the piezoelectric vibrator, an impact hammer, a drop of a metal weight or the like is utilized.
The impact hammer is extensively used in a wide field for the reasons such as its simpleness and the magnitude of impact energy. The vibration band is several tens kHz. It is also applied to the nondestructive testing of a long and large concrete structure. In such a concrete structure, in order to obtain the reflection wave, a wave having a certain wavelength or more is required. However, the reflection wave might be buried in a signal of the impact. It is necessary to suitably adjust the intensity of the impact to meet the purpose of measurement. Namely, this largely depends upon the experiences or a sense of the tester. On the other hand, it is difficult to always keep the vibration force constant. The waveform observed varies for every impact. This leads to the fluctuation in evaluation. The vibration band is about 1 kHz but it is impossible to control the frequency as desired. In particular, it is difficult to detect the reflection wave in a short distance due to the affect of its reverberation wave.
The operation of a supersonic wave testing method using the piezoelectric vibrator will now be described.
A pulse reflection method and a transmission method are known as one of the measurement methods using the supersonic wave.
The pulse reflection method and transmission method are a method in which the pulsational supersonic wave is introduced from the surface of the structure, a time period until the reflection wave thereof comes back or a time period until the transmission wave propagates is measured, the acoustic propagation speed or the thickness or the distance to the reflection surface of the measurement object is obtained from the time period to thereby infer the interior structure or the absence/presence of the damage of the measurement object. On the other hand, the resonance method is a method in which a wavelength of the supersonic wave to be introduced into the measurement object is continuously changed by sweeping the frequency of a piezoelectric type oscillator to measure the resonance frequency and the plate thickness is measured from the frequency.
Furthermore, as a method of measuring an acoustic propagation speed or a thickness of the measurement object, there is a sing around method in which the introduced supersonic wave pulse is detected by a wave receiver at the end face and the detected pulse is used as a trigger to repeat the oscillation of the supersonic wave pulse. According to this method, a pulse row is generated at a constant cycle. This cycle is identical with the delay time for the pulse to propagate through the measurement object. It is therefore possible to obtain the acoustic propagation speed or the thickness of the measurement object.
The operation of the impact elastic wave method using the impact hammer will now be described.
The impact elastic wave method is a method in which the hammer impact is given to the measurement object so that a proper vibration owned by the object per se is stimulated and utilized in measurement. This method may be widely applied to a concrete, building stone, a brick material, a timber structure, a laminated material, an underground buried object or the like and is widely used as a nondestructive inspection method owing to its easiness to carry out the test.
FIG. 20 is a view showing the constitution of the impact elastic wave method using the hammer. In the drawing, reference numeral 311 denotes a hammer, reference numeral 312 denotes an impact receiving sensor, reference numeral 313 denotes an elastic wave receiver, reference numeral 314 denotes a storage oscilloscope and reference numeral 315 denotes a measurement object.
The operation of the reflection wave measurement method by hammering will now be described. The impact receiving sensor 312 is applied to the measurement objective surface of the measurement object 315 and the hammer 311 impacts the surface. In order to enhance the precision of measurement at this time, it is necessary to be attentive and adjust the intensity of the impact depending upon the measurement purpose or the material of the measurement object 315 and make sure that the impact is given only once. The elastic wave introduced into the measurement object 315 by the impact advances through the interior of the object 315 to be measured while reflected at an abnormal portion such as the confronting surface of the measurement objective surface, an internal structure or a damage or a gap, so that a part thereof reaches the impact receiving sensor 312. The output of the impact receiving sensor 312 passes through a filter and the waveform of the frequency corresponding to the purpose of measurement is extracted. When the pulse wave by the impact is given as a trigger signal of the storage oscilloscope 314 to catch the reflection wave, the time period from the hammer impact time to the arrival of the reflection wave can be measured. Thus, the distance up to the surface where the reflection wave is generated can be obtained from the measured time and the acoustic speed of the material.
In the conventional diagnosis of a concrete structure, since the acoustic elastic wave is generated by using the hammer or the wave sensor for the supersonic wave as described above, there are the following problems.
Supersonic Wave Test Method
(1) The wave transmitter of the supersonic wave is suitable for outputting the acoustic elastic wave having a frequency of several tens kHz or more in view of the constitution of the vibrator of the piezoelectric type oscillator. For this reason, the method may be applied to the measurement for a material that is small in attenuation within a medium, such as metal, or a thin material. However, in case of a measurement object such as a concrete, where the attenuation within a medium is severe, the reached distance of transmission or reflection is short, and it is difficult to apply the method to the diagnosis of the long and large concrete structure.
(2) In order to identify the reflection wave, specialized knowledge is necessary. In particular, in case of a complicated shape or complicated internal structure, it is difficult to evaluate it.
(3) In the case where the propagation distance is long and the attenuation is severe or in the case where the reflection from the internal structure or the abnormal portion or the like of the measurement object is weak, the amplitude of the reflection wave to be detected would be minute in the same level as that of the noise. In such a case, the error would occur in detection of the reflection position or the speed.
(4) Any of the methods requires special signal processing or judgment from the shape of the reflection wave, demanding the special knowledge.
Impact Elastic Method
(1) The hammer is bounded when the vibration is applied. As a result, the vibration application is performed plural times in a short cycle. For this reason, the reflection wave is hidden in the vibration signal or the signal cannot be discriminated from the reflection wave. Accordingly, an accurate measurement it is difficult. In order to avoid these, the tester has to be trained well for vibration application.
(2) Because the intesity of the vibration is adjusted manually, a signal suitable for the measurement, that is, the intensity enough to attain the reflection is not obtained, or the vibration is too high to become the intended impact signal, for outranging the dynamic range of the amplifier or the accelerator, and instead a complicated frequency component is generated. If the frequency component is near the proper vibration of the structure, an erroneous measured value is presented.
(3) Namely, there is no reproducibility in vibration and measurement result fluctuates according to the degree of skill of the tester.
(4) In the case where the thickness of the structure is relatively thin, a main component of the vibration by the hammer is sometimes lower than the proper frequency of the structure. In this case, the reflection wave is hidden by the reverberation after the vibration application, and it would be difficult to detect the reflection wave.
(5) It is impossible to perform the vibration control such as changing the vibration application frequency as desired or applying vibration under the feedback.
The present invention has been made to solve the above-described problems, and an object of this invention is to provide a nondestructive testing apparatus for a concrete structure with which a highly reproducible, stable, highly precise test can be performed without necessity for the skill or experience of an operator and which is easy and does not particularly require a special knowledge for evaluation.